1. Field of the Invention
This invention relates to micromachined silicon sensors or Micro Electro Mechanical Systems (MEMS) mass flow sensing technology that measures the quality of gases. The present invention also relates to thermal mass flow sensors of such gases. This invention additionally provides the design and make of a micromachined mass flow sensor. The present invention specifically relates design and process of making the same for a mass flow sensor for preventing of liquid vapor or liquid condensation of such sensors that will be used to measure the flow at a highly humidified gas medium.
2. Description of the Related Art
Micromachined mass flow sensors (aka silicon flow sensors) are generally made on silicon wafers, and have been widely applied in the past decades in medical, automotive, and many other industries where clean and dry gas flow measurements are demanded for high accuracy, low cost and enhanced performances at a small form factor and low power. Examples are medical anesthesia gas control, personal ventilators, air intake of automotive electronic control units, and gas chromatography mass spectrometry. One of the earlier makes of silicon flow sensors is disclosed by Higashi et al. (Higashi. R. E. et al., Flow sensor, U.S. Pat. No. 4,501,144, Feb. 26, 1985) of Honeywell for a small footprint silicon flow sensor that has its binding wires to the control electronic interface exposed to the flow medium which limited the applications to clean, non-conductive and dry gases. Ueda et al. (Ueda. N. and Nozoe, S., Flow rate measuring device, US Patent Application 2008/0314140) and Fujiwara et al., (Fujiwara, T.; Nozoe, S. and Ueda, N., Flow velocity measuring device, U.S. Pat. No. 7,062,963) of Omron designed a complicated by-pass segregation channel to avoid the damages from impact of particles in the fluid with tiny particles as well as clogging of the flow channels. These mechanical package designs however did not change the basic performance of the silicon flow sensor as the conductive or high humidified fluids could easily destroy the sensor chip by shorting the wires. Additional clogging with liquid condensation would also take place in cases that the fluid has the liquid vapor or is highly humidified. Mayer et al. (Mayer. F. and Leaner, M., Method and sensor for measuring a mass flow, U.S. Pat. No. 6,550,324) teach an integrated MEMS mass flow sensor chip using thermal pile sensing elements and CMOS integrated signal processing circuitry that seals the wire from the contact of the flow fluids but limited the flow channel size to within 2 mm in diameters by the geometry of the sensing chip. In another disclosures by Hecht et al, (Hecht. H. et al., Method for correcting the output signal of an air mass meter, U.S. Pat. No. 5,668,313) and Wang et al., (Wang, G. et al., Micro Machined mass flow sensor and insertion type flow meters and manufacture methods, U.S. Pat. No. 7,536,908), the silicon mass flow sensors were designed without on chip electronics and the sensor size is elongated such that the wire connections to the electronics interface could be completed sealed at one end of the silicon flow sensor chip and the sensor could be packaged into a formality of a probe that could be inserted into a flow channel of arbitrary sizes that is calibrated together for the performance. However, because of the nature of the direct, contact of the silicon flow sensor chip with the flow fluids during the operation, the fluid with vapors or highly humidified gas flow medium will still significantly affect the flow readout as the flow medium characters would be significantly deviated, from those at the calibration. Application examples for these type of flow media are commonly seen in human respiratory, vaporized carbon dioxide for beverage and food, to name a few. Bonne and Satren (Bonne, U. and Satren, E., Sensor package for harsh environments, U.S. Pat. No. 6,911,894) and Mayer et al. (Mayer. F., Honing, R. and Vanna, S., Flow sensor, U.S. Pat. No. 6,813,944) revealed a similar structure that places the silicon flow sensor chip outside the flow channel to avoid the direct contact of the silicon flow sensor chip with the flow fluids. This structure can also be used in liquid fluid flow measurement therefore it is an effective approach for maintaining the sensor performance in a fluid with vapors or highly humidified flow medium. Nonetheless, the design limits the flow channel dimensions to be within, a few mini-meters because of the small foot print of silicon flow sensors, which in return restrict the applications only for very small flow measurement applications. In addition, because of the small power of the silicon flow sensors, the package or flow channel material directly in contact with the sensor must have superior thermal transfer properties that also limits the package options and results in a high cost for the products. Alternative operation of the silicon sensor at an elevated current or high power of the micro-heater to avoid the sensor deviation in performance for flow fluids with vapors or high humidity as proposed for the thin film or hot wires flow sensors (Eirnsnf, K., Ullrich, K. and Muziol, M., Flow sensor element and its self-cleaning, U.S. Pat. No. 7,739,908) is often difficult since the high current or high power could expose the silicon flow sensors to volatility during performance. Further, the continued operation of the silicon flow sensor in a fluid with vapors or high humidity would eventually leads to silicon flow sensor surface condensation as the desired low micro-heater power would not be sufficient to expel the vapor accumulation that would result in sensor malfunction or significant deviations in flow read out.
Therefore it is desired to have a completed new design or disclosure of a silicon flow sensor that shall perform in a fluid with vapors or high humidity. This sensor shall be able to continue working in such environments and maintaining good accuracy and reliability. The desired silicon flow sensor shall also keep its small foot prints while could be operated at a low power configuration. Further there should not be any limitations for the desired silicon flow sensors that shall be able to be packaged for arbitrary flow channel sizes and performed in any fluid properties.